MRAM magnetic memories have experienced a resurge of interest with the development of magnetic junction tunnels having strong magnetoresistance at ambient temperature. MRAM memories comprise several memory cells. Said memory cells are generally magnetic devices that comprise:                a magnetic layer known as “reference layer” that has a magnetisation, the direction of which is fixed;        a magnetic layer known as “storage layer” that has a magnetisation, the direction of which is variable and which can orient itself either parallel or antiparallel to the direction of magnetisation of the reference layer;        a spacer made of an insulating or semi-conductor material that separates the reference layer and the storage layer.        
The document FR2817999 describes for example such a magnetic device. Said magnetic device has two modes of operation: a “reading” mode and a “writing” mode. In writing mode, a flow of electrons or a magnetic field is sent through the layers so as to cause the reversal of the direction of magnetisation of the storage layer, which then becomes parallel or antiparallel to the direction of magnetisation of the reference layer. Depending on whether the direction of magnetisation of the storage layer is parallel or antiparallel to the direction of magnetisation of the reference layer, a “1” or a “0” is stored in the storage layer.
In reading mode, a flow of electrons is injected through the magnetic device so as to read its resistance. When the directions of magnetisation of the reference layer and of the storage layer are parallel, the resistance of the junction is low, whereas when the directions of magnetisation of the reference and storage layers are antiparallel, the resistance of the junction is high. By comparison with a reference resistance, the value stored in the storage layer (“0” or “1”) may be determined.
The document FR2924851 furthermore proposes adding to the device described previously an antiferromagnetic layer in contact with the storage layer. In reading mode, said antiferromagnetic layer makes it possible to fix the direction of magnetisation of the storage layer so that the information stored in the storage layer does not vary. In writing mode on the other hand, the antiferromagnetic layer is heated so that it becomes paramagnetic or at least that the temperature thereof exceeds the temperature known as blocking temperature of the antiferromagnetic layer. This phenomenon, known as exchange bias, is based on the following principle: when a ferromagnetic material is put in contact with an antiferromagnetic material, there may be appearance of unidirectional anisotropy: the hysteresis loop of the ferromagnetic material then has a field shift as if a constant field was superimposed on the exterior field applied; said constant field results from the exchange interaction through the interface with the antiferromagnetic material.
The blocking temperature is in general below the Néel temperature of the antiferromagnetic material but it approaches more and more the Néel temperature as the duration of the heating is reduced towards times of the order of several nanoseconds. The Néel temperature of the antiferromagnetic material is the temperature at which antiferromagnetic order disappears and above which the material behaves like a paramagnetic material. When the temperature of the antiferromagnetic layer exceeds the blocking temperature, the direction of magnetisation of the storage layer may then be modified since it is no longer trapped by the antiferromagnetic layer. Once the direction of magnetisation of the storage layer has been modified, the heating of the antiferromagnetic layer is stopped. The antiferromagnetic layer then again becomes antiferromagnetic. The direction of magnetisation of the storage layer is then trapped in the direction in which it was found at the end of the writing process. Magnetic devices that comprise such an antiferromagnetic layer and implement a temporary heating of said layer at the time of writing of the information are part of devices known as “thermally assisted writing” devices.
Magnetic devices with thermally assisted writing are advantageous because they make it possible to reduce the risk of accidental writing during the reading of the information contained in the storage layer. Furthermore, they have much better retention than devices not implementing thermal assistance, in other words they have a better capacity to preserve written information over time.
Nevertheless, in these thermally assisted writing devices, an important density of structural defects is observed due to the structural incompatibility between the antiferromagnetic layer, which generally has a centred-face cubic crystallographic structure, and the storage layer, which generally has a centred cubic crystallographic structure when it is in contact with an MgO tunnel barrier. Said structural defects have a direct impact on the trapping quality of the storage layer by the antiferromagnetic layer.
To overcome these drawbacks, the document FR2924851 proposes adding in the storage layer an amorphous or quasi-amorphous layer, for example made of tantalum, as well as a ferromagnetic layer with centred-face cubic crystallographic structure, for example made of permalloy NiFe. The addition of said layers makes it possible to make a structural transition between the antiferromagnetic layer and the layers that have a centred cubic crystallographic structure.
However, magnetic devices with thermally assisted writing of the prior art have numerous drawbacks when the thickness of the antiferromagnetic layer or the lateral size of said devices is reduced.
Thus, as mentioned previously, the antiferromagnetic layers used in the thermally assisted writing devices to trap the storage layers crystallise in the cubic network with centred faces (case of the compounds FeMn and IrMn). The growth of said materials generally takes place along the axis [111] of the dense planes, growth axis all the more pronounced when the buffer layers follow the same axis, which is the case of NiFe for example, and more generally of alloys based on Co formed by cathodic sputtering. In the planes (111), the magnetic moments composing the antiferromagnetic network are oriented so that the resulting total moment is zero: these compounds are known as antiferromagnetic materials with compensated moments (see J. Phys. Soc. Jpn 21, 1281 (1966) and J. Appl. Phys. 75, 6659 (1994) for the compound FeMn and J. Appl. Phys. 86, 3853 (1999) for IrMn).
This set of non-compensated moments should theoretically lead to an absence of exchange bias between the ferromagnetic layer and the antiferromagnetic layer but there exists in the polycrystalline antiferromagnetic layers, as mentioned above, a mozaicity which results in a distribution of the axes [111] around the z axis perpendicular to the plane of the layers, as shown in FIG. 1, which schematically represents a sectional view of the interface between a ferromagnetic layer 100 and a granular antiferromagnetic layer 200 composed of a plurality of grains (here two grains 200A and 200B are represented separated by a grain joint 300). Inside each antiferromagnetic grain, where the growth is coherent, the directions of magnetisation of the antiferromagnetic layer 200 thus form a completely compensated network oriented on average parallel to the interfaces 400A and 400B between the ferromagnetic layer 100 and the antiferromagnetic layer 200, but distributed over the whole of the layer due to the distributions of the growth axes [111]. Thus, the exchange bias between the ferromagnetic layer 100 and the antiferromagnetic layer 200 takes place essentially via the magnetic moments (designated equally well hereafter magnetisation vectors or magnetisations) non-compensated at the interface, that are found at the grain joints for example, or at the level of the atomic steps (cf. the atomic step 500 between the interfaces 400A and 400B) composing the interfacial rugosity, since everywhere else (where the growth of the first antiferromagnetic layer is coherent), the antiferromagnetic interfacial moments have a zero outcome and induce little or even no bias on average with the ferromagnetic layer 100.
Moreover, the antiferromagnetic moments orient themselves preferentially along certain easy axes, said anisotropy being characterised by the constant anisotropy noted KAF of the antiferromagnetic layer expressed in erg/cm3.
If a single antiferromagnetic grain is considered, having a cylindrical geometry, the thickness is tAF and the surface area is S, in order to observe a shift of the hysteresis loop of the ferromagnetic layer in contact with said grain, it is necessary that KAF×S×tAF>J0×S (1) where J0 represents the energy of interaction at the interface between the ferromagnetic layer and the antiferromagnetic layer expressed in erg/cm2. This inequality (1) means that the antiferromagnetic moments of the grain Must not follow the ferromagnetic moments during the rotation thereof to contribute to the exchange anisotropy. Consequently, with a view to miniaturisation capable of inducing a reduction in the thicknesses of the devices, it may be observed that the bias effect is no longer observed below a critical thickness. In addition, it is known that the blocking temperature of antiferromagnetic layers diminishes with the thickness thereof, and that it is necessary to attain this blocking temperature to modify the direction of magnetisation of the ferromagnetic storage layer (J. Appl. Phys. 83, 7216 (1998)). Said reduction in the blocking temperature (enabling energy savings during writing phases) is thus limited by the existence of said critical thickness below which the bias effect is no longer observed. The fact of reducing the thicknesses of antiferromagnetic material, the cost of which is high, makes it possible moreover to make production savings.
Furthermore, in a granular polycrystalline antiferromagnetic layer, the size of the grains is widely distributed. For purely illustrative purposes, FIG. 2 schematically represents a plurality of grains of different size in a structure comprising an antiferromagnetic layer AF and a ferromagnetic layer F. In this figure, three grains having decreasing surfaces from S1 to S3 are illustrated. In accordance with the inequality (1) presented in the preceding paragraph, it is considered that only the largest grains contribute to the exchange field. It is known that the energy of interaction is dependent on the surface area of the grains: the larger said surface area, the lower the energy of interaction, and vice versa (J. Appl. Phys. 83, 6888 (1998)). Thus, in the case of FIG. 2, exchange energies J1 to J3 corresponding to the surfaces S1 to S3 are less and less high such that the effect of the exchange anisotropy of the smallest grains is less important.
When the size of the thermally assisted writing devices of the prior art is reduced, the stability of the antiferromagnetic layer is reduced, as is the trapping of the storage layer by the antiferromagnetic layer. In fact, the antiferromagnetic layer has a polycrystalline granular structure and the grains constituting the layer are weakly magnetically coupled together. Yet, when the size of the magnetic device is reduced, the proportion of the grains of the antiferromagnetic layer that are situated at the periphery of the antiferromagnetic layer is increased. In addition, said peripheral grains are in part eaten away by the etching process, which has the effect of making their magnetisation less stable. They may even be qualified as magnetically unstable when the eaten away part exceeds a certain percentage of the eaten away part of the initial surface of the grain. As the size of the device is reduced, the size distribution of the grains constituting the antiferromagnetic layer tends to widen. This results in very important fluctuations of trapping properties from one device to the next, and in the example of magnetic memories from one memory point to the next. Thus, when devices of the prior art of small dimensions are etched, the volume of said peripheral grains is reduced, which leads to a reduction in the magnetic coherence of the antiferromagnetic layer linked in particular to the presence of small grains. Said reduction in the magnetic coherence of the antiferromagnetic layer brings about a more or less significant reduction in the trapping quality of the storage layer by the antiferromagnetic layer and an increase in the dispersion of the trapping properties from one point to the next.